In the prior art, there are many ways of making a document safe from alteration. Prior art techniques, for the most part, are based upon the utilization of chemical formulations which are or may be employed as either an overlay on the top surface of the document to be protected or as an overprinted area on such documents.
With the advent of xerographic color copiers such as the Xerox L-6500 color copier or similar machines, the problem of nefarious reproductions has dramatically increased. The quality of the color reproduction at this point in the art is such that it is very difficult, often impossible, to discern whether the copied document is the original or a color copy. The reproduction of checks, stock certificates, automobile title instruments, etc., can be readily accomplished.
In copending application Ser. No. 766,590, filed Feb. 8, 1977, entitled "Protection System for Documents," incorporated herein fully by reference, there has been disclosed a system for protection of documents which employs a masked warning mark which, when copied, appears on the copy due to its effective color density being above the color reproductive threshold density of the copier.
In another copending application, Ser. No. 809,261, filed June 23, 1977, for "Protected Document Bearing Watermark and Method of Making Same," which is incorporated herein by reference, another method for protecting documents is disclosed. This latter application employs a cancellation phrase pattern of two or more different screen tones.
The inventions described and claimed in these applications operate satisfactorily. The present application describes and claims still other methods of making documents secure.